The present invention relates to bipolar transistors capable of being used as high power transistors using radio frequencies, and to methods for fabricating the same.
Group III–V compound semiconductors of, for example, gallium arsenide (GaAs) or indium phosphorus (InP) have the following advantages. For example, the Group III–V compound semiconductors exhibit excellent electrical characteristics in, for example, electron mobility and electron saturation velocity, as compared to silicon (Si)-based semiconductor materials. In addition, the Group III–V compound semiconductors can be used in designing semiconductor devices with desired energy band structures utilizing a heterojunction or can be used as semi-insulating substrates.
In particular, a heterojunction bipolar transistor (HBT) which uses, for its emitter layer, a Group III–V compound semiconductor having a wider band gap than the base layer exhibits characteristics such as being operable with a single power source, a high degree of efficiency in adding power, and an excellent linearity of power amplification. Accordingly, such HBTs have been widely used as high power transistors for cellular phones.
An aluminum gallium arsenide (AlGaAs)/GaAs-based HBT using p-type GaAs and n-type AlGaAs for its base and emitter layers, respectively, and an indium gallium phosphorus (InGaP)/GaAs-based HBT using p-type GaAs and n-type InGaP for its base and emitter layers, respectively, are known as conventional HBTs.
FIG. 10A shows a cross-sectional structure of a known InGaP/GaAs-based HBT. As shown in FIG. 10A, a collector contact layer 102 of high-concentration n-type GaAs, a collector layer 103 of low-concentration n-type GaAs, a base layer 104 of p-type GaAs, an emitter layer 105 of n-type InGaP, and an emitter contact layer 106 made of a stack of n-type emitter layers, are stacked in this order over a substrate 101 of GaAs.
An emitter electrode 107 is formed on the emitter contact layer 106. The emitter layer 105 is formed on the base layer 104 to have a mesa configuration. Base electrodes 108 are formed on the base layer 104 at the sides of the emitter layer 105. A collector electrode 109 is formed on the collector contact layer 102 at a side of the collector layer 103.
In the known HBT, since the emitter layer 105 is made of InGaP having a wider bandgap than the base layer 104, the backflow of holes from the base layer 104 to the emitter layer 105 can be suppressed. Therefore, the thickness of the base layer 104 can be reduced and, at the same time, the concentration of the p-type impurity can be increased. Accordingly, it is possible to increase flow time of electrons in the base layer 104, while suppressing the base resistance. As a result, the known HBT can be used as a power amplifier operable at high speed.
Now, in the known HBT, the emitter layer 105 with the mesa configuration includes: an emitter region 105a located under the emitter contact layer 106 and actually serving as an emitter; and a surface-protection region 105b connected to the emitter region 105a. The base layer 104 is divided into an intrinsic base region 104a located under the emitter region 105a and actually serving as a base and an extrinsic base region 104b connecting the base electrodes 108 and the intrinsic base region 104a together.
The surface-protection region 105b has a function of preventing recombination of holes and electrons injected from the emitter electrode 107 into the emitter region 105a through the emitter contact layer 106 in the surface of the extrinsic base region 104b. 
FIG. 10B shows the emitter layer 105 and the peripheral portion thereof in FIG. 10A in an enlarged manner, by overlaying equivalent circuit symbols thereon. As shown in FIG. 10B, a positive direct current DC is input to each of the base electrodes 108 together with an RF input signal RFIN, thereby using an amplified RF power of the input signal RFIN. In this case, the base layer 104 is doped with a p-type impurity at a high concentration and the base layer 104 serves as a resistance to the direct current DC and input signal REIN.
In the case where the known HBT is applied to a high power device, about 10 to 100 HBTs are connected in parallel, taking the HBT shown in FIG. 10A as one unit cell. However, there are cases where the degree of temperature rise differs among the HBTs because of variation in operating state or the like. In such cases, the ON voltage between the emitter and the base decreases in some of the HBTs under high temperatures, so that the emitter current increases, thus causing further temperature rise. As a result, operation of the high power device becomes thermally unstable.
To solve the problem, a configuration in which a resistance element for stabilizing operation which is called a ballast resistance is provided to a base input terminal in each of the HBTs is known.
FIG. 11 shows a circuit configuration of a known high power device in which a ballast resistance is provided in each of the HBTs. As shown in FIG. 11, to respective base terminals of bipolar transistors Q1 through Qn, a direct current DC is input via ballast resistances R1 through Rn and an input signal RFIN is input via input capacitances C1 through Cn.
With such a configuration, if current tends to be concentrated in one of the bipolar transistors, e.g., bipolar transistor Q1, voltage drop is caused by the ballast resistance R1. Accordingly, the voltage applied at the base layer decreases, thus making the current less concentrated. In addition, since the input signal RFIN is input to the base electrodes via the input capacitances C1 through Cn, the ballast resistances R1 through Rn cause no deterioration of the RF characteristic.
The high power device shown in FIG. 11 is obtained by forming the bipolar transistors Q1 through Qn having the same configuration as that of the HBT shown in FIG. 10A. In this case, the ballast resistances R1 through Rn are formed in part of the substrate other than a region in which an HBT is to be formed by using a thin film of a metal or a semiconductor material, and the input capacitances C1 through Cn are formed by using a capacitive insulating film of, for example, silicon nitride (SiN) and a conductor film of a metal.
However, in the known HBT, provision of the surface-protection region 105b increases the distance between the base electrodes 108 and the emitter electrode 107, so that the base resistance increases. Accordingly, current of an input signal input from the base electrodes 108 decreases to a larger extent, resulting in deterioration in the RF characteristic of the HBT.
In addition, as in the known high power device, provision of the input capacitances C1 through Cn, and the ballast resistances R1 through Rn to the bipolar transistors Q1 through Qn needs securing an input capacitance region and a ballast resistance region as well as an HBT region. This increases the chip area, thus increasing the cost for a chip. In particular, if nitride silicon is used for a capacitive insulating film, a rectangular region with sides of 10 μm or more is required for every HBT in order to secure a capacitive value required as an input capacitance, resulting in that the cost for a chip remarkably increases. In addition, it is also necessary to form ballast resistances and input capacitances after forming HBTs, so that the manufacturing cost increases.